1. Field of the Invention
The present invention relates to electrical potentiometric devices, and to electronic ratiometric devices, more specifically, to electronic devices developed for the purpose of replacing or simulating the electrical characteristics of a potentiometric device. Further, the present invention relates to sensors having a potentiometric electrical connection, or ratiometric output signal.
2. Description of the Prior Art
A Prior Art potentiometric device, also called a potentiometer, is a three-terminal electrically resistive device. See FIG. 1, PRIOR ART. Such a potentiometer typically comprises at least a resistive element 1, a wiper 2, that makes electrical contact to the resistive element, and three electrical terminals 6, 7, 8, for connection into an electrical circuit. An optional fourth electrical terminal may also be included to allow a ground or case connection (optional fourth terminal not shown in figures). In a rotary potentiometer, such as the one shown in FIG. 1, the wiper moves in an arc that pivots from a wiper pivot 3, near one end. Linear potentiometers are also in common use, with the resistive element being made approximately in a straight line (not shown) rather than in an arc as shown in FIG. 1.
A power source is connected to the potentiometer such that a power supply voltage appears across first and third terminals 6, 8, of FIG. 1, for example, zero and ten volts DC (direct current). There exists a constant electrical resistance between the first and third terminals, for example, five thousand ohms. A current flows through the resistance, which is two mA (milliamperes) in the example (i.e. ten volts divided by five thousand ohms). In the example, the input power would be two mA multiplied by ten volts, or twenty mW (milliwatts). The wiper 2, of a potentiometer provides an output voltage at second terminal 7. The wiper makes physical contact with the resistive element 1, in at least one point. The mechanical configuration of the wiper is such that its point of contact with the resistive element is movable along at least a portion of the length of the resistive element. As the point of contact between the wiper and the resistive element moves along the arc or the length of the resistive element, an output voltage appearing on the second terminal 7, varies as a percentage of the voltage across the resistive element 1, and in proportion to the relative position of the wiper 2, along the resistive element.
The resistive element 1, typically comprises a substrate of ceramic or other mechanically suitable electrically insulative material, having at least one surface that is coated with a thin layer of electrically resistive material. Typical power supply voltages for a potentiometer are 5, 10 or 24 volts DC, but other voltages may be used. It is uncommon for the power supply voltage to be above 30 volts DC. Typical resistances of the resistive element are one or two thousand ohms when used with a five volt power supply, five thousand ohms when used with a ten volt power supply, or ten thousand ohms when used with a twenty four volt power supply. It is not desirable to use a lower resistance element, such as one thousand ohms, with a higher power supply voltage, such as 10 or 24 volts, due to the higher current that would be drawn from the power source, and the resulting increase in power dissipation of the potentiometer.
As shown in FIG. 1, first terminal 6, is connected to resistive element 1, at a location approximately along one end of the resistive element, forming a first resistive element connection 4. Likewise, a third terminal 8, is connected to resistive element 1, at a location approximately along another end of the resistive element, forming a second resistive element connection 5. It is desirable that wiper 2, remain in constant contact with resistive element 1, and to be prevented from riding up onto the areas of connections 4, and 5. This will prolong the life of the wiper 2, and also will help to reduce intermittent loss of contact between the wiper 2, and the resistive element 1. Therefore, motion of the wiper is commonly restricted to a range slightly less than that required to obtain output voltages equal to the power supply voltage. For example, with terminals 6, and 8, connected to 10 and 0 volts DC (Direct Current), respectively, full wiper motion over the mentioned restricted range will result in output voltages up to approximately 9.900 volts DC, and down to approximately 0.100 volts DC (or, when connected to 5 and 0 volts DC, output voltages can be obtained of up to approximately 4.950 volts DC and down to 0.050 volts DC, respectively).
The main advantage of using a potentiometer as a means for providing a variable voltage is its simplicity. The major disadvantage of a potentiometer is that mechanical contact between the wiper and the resistive element constitutes a mechanism for wear. Wear resulting from repeated mechanical motion of the wiper normally limits the lifetime of a potentiometer. End of service life of a potentiometer typically occurs when wearing of the surface of the resistive element causes erratic voltages to appear on its output (represented here as second terminal 7), due to several factors, including buildup of particles that have been scraped from the resistive element by the wiper movement, partially bare spots where the coating of the resistive element has been removed from the underlying substrate, as well as changing the contact properties of the surface of the resistive element.
It is common in the prior art for other types of electronic devices to be developed in attempts to simulate the simplicity of wiring that is inherent with a potentiometer, but having other undesirable attributes which limit such an electronic device from being directly interchangeable with a potentiometer. Some of the undesirable attributes of such prior art electronic devices include a higher current draw from the power source, a more narrow range of allowable power supply voltage, and a more narrow range of output voltage available. Many such prior art electronic devices require a power supply voltage in the narrow range of 5.0 volts +/−0.5 volts, and provide an output voltage range of 10% to 90% of the power supply voltage for indications of zero and full scale, respectively. Such prior art electronic devices typically draw from 10 to 150 milliamperes of current from the power source.
A potentiometer is commonly employed to provide a variable output voltage in response to a physical parameter being measured (that is, in response to a parameter). Such a potentiometer is often configured as a position-measuring sensor, but potentiometric devices can be used to sense other parameters such as pressure, flow, etc. when coupled to a mechanical system that provides a mechanical motion proportional to the parameter.
The physical parameter can be mechanically coupled to the potentiometer directly, or transduced from one form of mechanical energy or motion into another as appropriate for the given parameter. For example, a diaphragm or bellows can be used to transduce a pressure measurand into a linear motion. The linear motion can be coupled to a linear potentiometer. Such a potentiometric device or combination of potentiometer and transducer can be called a potentiometric sensor.
A potentiometric sensor with a wiper that contacts and rubs along a surface of the resistive element is called a “contact-type” sensor, that is, the wiper makes mechanical contact with the resistive element. Because the typical potentiometer has only three wires, it is relatively simple to connect into an electrical system, and is also easily understood.
Various electronic devices, and especially sensors, have been developed which simulate the function of a potentiometric device to some extent. The output of such a device or sensor is typically called ratiometric. In a device having a ratiometric output, an output voltage is developed that is similar to an output voltage developed in a potentiometric sensor, in that the output voltage is a percentage of an applied power supply voltage. Many ratiometric electronic sensors have an advantage over an actual potentiometric contact-type sensor, because they can utilize capacitive, inductive, or magnetic sensing, for example, and thereby make their measurement without physical contact among moving and non-moving members comprising the device or sensor. This type of sensor arrangement is called a non-contact ratiometric sensor. This eliminates mechanical wear, and can provide an increase in the service lifetime of the sensor.
By virtue of having a three-wire electrical connection, a non-contact ratiometric sensor as described above can sometimes be used as a replacement for a potentiometric contact-type sensor. A typical sensor of this type uses an electronic circuit that requires an input voltage of 4.5 to 5.5 volts DC at a current level of between 10 mA and 150 mA, and produces an output voltage in the range of 10% to 90% of the power supply voltage in response to a 0% to 100% range of a measurand. For example, with a 0 to 1 inch linear position sensor having a power supply voltage of 5.0 volts DC, an output voltage range would typically be 0.5 to 4.5 volts DC for positions from 0 inches to 1 inch. Although this can be accommodated by some types of receiving electronics with appropriate adjustments, it is not serviceable as a direct replacement of a potentiometric contact-type sensor in many applications.
To the contrary, the present invention teaches an apparatus which can directly replace a potentiometric contact-type sensor in virtually all applications, while preserving its desirable performance characteristics and simplicity of wiring.